In the traditional casting process, molten metal is poured into a mold and solidifies, or freezes, through a loss of heat to the mold. When enough heat has been lost from the metal so that it has frozen, the resulting product, i.e., a casting, can support its own weight. The casting is then removed from the mold.
Different types of molds of the prior art offer certain advantages. For example, green sand molds are composed of an aggregate, sand, that is held together with a binder such as a mixture of clay and water. These molds may be manufactured rapidly, e.g., in ten (10) seconds for simple molds in an automated mold making plant. In addition, the sand can be recycled for further use relatively easily.
Other sand molds often use resin based chemical binders that possess high dimensional accuracy and high hardness. Such resin-bonded sand molds take somewhat longer to manufacture than green sand molds because a curing reaction must take place for the binder to become effective and allow formation of the mold. As in clay-bonded molds, the sand can often be recycled, although with some treatment to remove the resin.
In addition to relatively quick and economical manufacture, sand molds also have high productivity. A sand mold can be set aside after the molten metal has been poured to allow it to cool and solidify, allowing other molds to be poured.
The sand that is used as an aggregate in sand molding is most commonly silica. However, other minerals have been used to avoid the undesirable transition from alpha quartz to beta quartz at about 570 degrees Celsius (° C.), or 1,058 degrees Fahrenheit (° F.), that include olivine, chromite and zircon. These minerals possess certain disadvantages, as olivine is often variable in its chemistry, leading to problems of uniform control with chemical binders. Chromite is typically crushed, creating angular grains that lead to a poor surface finish on the casting and rapid wear of tooling. Zircon is heavy, increasing the demands on equipment that is used to form and handle a mold and causing rapid tool wear.
In addition the disadvantages created by the unique aspects of silica and alternative minerals, sand molds with clay and chemical binders typically do not allow rapid cooling of the molten metal due to their relatively low thermal conductivity. Rapid cooling of the molten metal is often desirable, as it is known in the art that with such cooling the mechanical properties of the casting are improved. In addition, rapid cooling allows the retention of more of the alloying elements in solution, thereby introducing the possibility of eliminating subsequent solution treatment, which saves time and expense. The elimination of solution treatment prevents the quench that typically follows, removing the problems of distortion and residual stress in the casting that are caused by the quench.
As an alternative to sand molds, molds made of metal or semi-permanent molds or molds with chills are sometimes used. These metal molds are particularly advantageous because their relatively high thermal conductivity allows the cast molten metal to cool and solidify quickly, leading to advantageous mechanical properties in the casting. For example, a particular casting process known as pressure die casting utilizes metal molds and is known to have a rapid solidification rate. Such a rapid rate of solidification is indicated by the presence of fine dendrite arm spacing (DAS) in the casting. As known in the art, the faster the solidification rate, the smaller the DAS. However, pressure die casting often allows the formation of defects in a cast part because extreme surface turbulence occurs in the molten metal during the filling of the mold.
Moreover, all molds made from metal possess a significant economic disadvantage. Because the casting must freeze before it can be removed from the mold, multiple metal molds must be used to achieve high productivity. The need for multiple molds in permanent mold casting increases the cost of tooling and typically results in costs for tooling that are at least five times more than those associated with sand molds.
As a result, it is desirable to develop a casting process and related apparatus that have the advantage of rapid solidification of metal molds, while also having the lower costs, high productivity and reclaim-ability associated with sand molds.